1. Field of the Invention
The present invention relates to a light-emission modulation scheme, and, in particular, to an optimum pulse modulation signal generation circuit for modulating an optical output of a light source, a semiconductor laser modulation device equipped with it, an optical scanning device, and an image formation device, applied to a laser printer, an LED printer, an optical disk device, a digital copier, an optical-communications device, etc., employing the light-emission modulation scheme.
2. Description of the Related Art
As a type of modulating an optical output of a light source, there are a power modulation type of modulating the amount of light itself, a pulse-width modulation type of modulating the emission time of the light, a power and pulse-width combined modulation type. Generally, the pulse-width modulation type is commonly used. There, a triangular wave or a saw-tooth wave corresponding to each pulse generating period is generated, it is compared with an analog video signal using a comparator, a pulse width modulation signal is generated. Alternatively, delay pulses may be generated by using a high frequency clock signal and performing frequency dividing on the clock signal in a digital manner, and a pulse width modulation signal is generated through performance of logical sum or logical product thereon.
In recent years, in a laser printer, a digital copier, and another image formation device, a further improvement in operation speed is desired. However, when the above-mentioned triangular wave or saw-tooth wave is used, it is difficult to improve operation speed while securing linearity/reproducibility of the triangular wave or saw-tooth wave.
In case applying a scheme of performing frequency dividing in a digital manner a high frequency clock signal, the highest operation frequency depends on the device applied, and has a problem in that it is difficult to improve the operation speed while securing the gray scale characteristics of an output image. For example, in order to achieve 256-step gray-scale modulation by using a pixel clock signal of 50 MHz, it is difficult to provide a triangle wave or saw-tooth wave having a satisfactorily linearity and swing in a period of 20 ns. In case of digital frequency dividing scheme, it is difficult to provide a clock signal having a frequency of 50 MHzxc3x97256=12.8 GHz.
An object of the present invention is to provide a pulse modulation signal generation circuit which enables generation of a pulse modulation signal of a desired pattern arbitrarily with a simple configuration, and, thereby, even in case the operation frequency is very high, fine gray-scale characteristics can be achieved on an output image.
A pulse modulation signal generating circuit according to the present invention includes:
a clock generating part (11) generating a high-frequency clock signal having a frequency higher than that of a pixel clock frequency; and
a serial modulation signal generating part (13) generating a serial modulation signal having a serial pulse sequence based on the high-frequency clock signal,
wherein light emission is modulated according to the serial modulation signal, and, thus, each pixel of an image is formed according at the pixel clock frequency.
The pulse modulation signal generating circuit may further include a modulation data generating part (12) generating modulation data comprising a predetermined bit pattern according to given image data, the serial modulation signal generating part generating the serial modulation signal based on the modulation data.
The above-mentioned modulation data generating part may include a look-up table (122) for converting given image data into the corresponding modulation data.
According to the present invention, no complex configuration is needed for generating a predetermined pulse pattern, and the pulse modulation signal generation circuit can achieve a fine gray scale on an image with a simple configuration while a speed of operation is high. Moreover, it becomes possible to form an image with an arbitrarily time interval without using a periodic pixel clock which determines 1 dot or 1 pixel by applying the above-mentioned configuration to an image formation device.
Moreover, a semiconductor laser modulation device, an optical scanning device, and an image formation device of small size, low cost, and power saving can be provided by making the pulse modulation part and high frequency clock generation part into an integrated circuit in one chip.
An exposure method according to the present invention includes the steps of:
a) driving a light-emitting unit according to modulation signal; and
b) exposing a photoconductor while scanning it with a laser beam emitted by the light-emitting unit,
wherein:
the modulation signal comprises a pulse sequence; and
an exposure energy distribution in which the photoconductor is exposed is determined as a result of control of both a pulse width and a pulse pattern of the pulse sequence.
There, as a result of control of the exposure energy distribution, a density of a latent image formed on the photoconductor may be controlled on each position/pixel.
The exposure energy distribution may thus be controlled not only by control of total light-emission time interval during each unit time or each pixel but also by control of light-emission timing there.
Thus, by controlling the exposure energy distribution, it becomes possible to make steep the rising/decaying part of the exposure energy distribution as shown in FIG. 46, and to easily control the linearly in width of the exposure energy distribution (namely, the diameter of a relevant dot in the image thus formed) by changing the pulse width and the pulse pattern in the optical modulation signal.
Furthermore, as shown in FIGS. 47 and 48, it becomes possible to control image density/gray scale (i.e., the diameter of a relevant dot for every pixel) still more finely than the frequency of the clock pulse of the image clock (pixel clock) signal.
Specifically, in the example of FIGS. 47 and 48, although the frequency of the image clock signal has eight periods per pixel, it becomes possible achieve total 19 steps of the exposure energy distribution thus beyond twice thereof, and, achieve densities/gray scale (dot diameters) in the same number as a result adopting a pulse pattern like this.